1. Field of the Invention
The present invention relates to an excitation controller and an excitation control method for stabilizing voltage in an electric power system.
2. Description of Related Art
FIG. 8 is a block diagram showing a conventional excitation controller disclosed in Japanese patent publication No. Hei. 10-2809833. In FIG. 8, the reference numeral 1 designates a synchronous machine; 2 designates a transformer; 3 designates a breaker; 4 designates a transmission line; 5 designates a transmission bus of a power plant; 6 designates a potential transformer (called "PT" from now on) for detecting an output terminal voltage V.sub.G of the synchronous machine 1; 7 designates a voltage setter for setting the output terminal reference voltage r.sub.G of the synchronous machine 1; 8 designates a subtracter for producing a deviation signal by subtracting the output terminal voltage V.sub.G from the reference voltage r.sub.G set by the voltage setter 7; and 9 designates a reduced gain circuit for multiplying the deviation signal fed from the subtracter 8 by a gain .beta..
The reference numeral 10 designates a potential device (called "PD" from now on) for detecting the transmission voltage V.sub.H on the transmission bus 5; 11 designates a high side voltage setter for setting a reference voltage r.sub.H of the transmission bus 5; 12 designates a subtracter for producing a deviation signal by subtracting the transmission voltage V.sub.H from the reference voltage r.sub.H set by the high side voltage setter 11; 13 designates a high side voltage control gain circuit for multiplying the deviation signal fed from the subtracter 12 by a gain K.sub.H ; 14 designates an adder for adding the multiplication result of the reduced gain circuit 9 and the multiplication result of the high side voltage control gain circuit 13; 15 designates an automatic voltage regulator (called "AVR" from now on) for controlling the rectifying timing of an exciter 16 using the addition result of the adder 14 as an input condition; 16 designates the exciter for supplying the field current to a field winding 17 of the synchronous machine 1 under the command of the AVR 15; and 17 designates the field winding of the synchronous machine 1.
Next, the operation of the conventional excitation controller will be described.
First, the PT 6 detects the output terminal voltage V.sub.G of the synchronous machine 1. Then, the subtracter 8 subtracts the output terminal voltage V.sub.G from the reference voltage r.sub.G set by the voltage setter 7, and outputs its subtraction result as the deviation signal, and the reduced gain circuit 9 multiplies the deviation signal by the gain .beta..
On the other hand, the PD 10 detects the transmission voltage V.sub.H of the transmission bus 5. Then, the subtracter 12 subtracts the transmission voltage V.sub.H from the reference voltage r.sub.H set by the high side voltage setter 11, and outputs its subtraction result as the deviation signal. The high side voltage control gain circuit 13 multiplies the deviation signal by the gain K.sub.H.
Subsequently, the adder 14 adds the multiplication result of the reduced gain circuit 9 and the multiplication result of the high side voltage control gain circuit 13, and the AVR 15 generates a timing signal for controlling the rectifying timing of the exciter 16 using the addition result of the adder 14 as the input condition of the following transfer function.
transfer function=K.multidot.(1+T.sub.LD.multidot.S)/(1+T.sub.LG.multidot.S) PA1 where, K is the gain constant; PA1 T.sub.LD and T.sub.LG are time constants; and PA1 S is the Laplace operator.
In response to the timing signal fed from the AVR 15, the exciter 16 supplies the field current to the field winding 17 of the synchronous machine 1.
Incidentally, if the addition result of the adder 14 is positive, the field current supplied to the field winding 17 is increased so that the output terminal voltage V.sub.G of the synchronous machine 1 increases, whereas if the addition result of the adder 14 is negative, the field current supplied to the field winding 17 is decreased so that the output terminal voltage V.sub.G of the synchronous machine 1 decreases.
Thus, the voltage on the transmission bus 5 is maintained at a fixed value, and hence, even if an accident takes place on the transmission line 4, the voltage drop of the entire transmission system can be alleviated, which makes it possible to markedly improve the voltage stability.
With the foregoing configuration, the conventional excitation controller can maintain the transmission voltage V.sub.H on the transmission bus 5 at the fixed value. However, it has a problem of increasing the manufacturing cost because of the expensive PD 10 which is required for detecting the transmission voltage V.sub.H of the transmission bus 5.
Furthermore, since an excitation control cubicle, on which the AVR 15 and adder 14 are mounted, is usually installed far from the transmission bus 5, a long cable connecting the excitation control cubicle and the transmission bus 5 is needed and is susceptible to noise, and this presents the problem of reducing the reliability of the system.